It is known a painting head essentially consisting of a parallelepiped container, with a front wall and a rear wall, parallel to each other, in which an inlet aperture and respectively an outlet aperture are made, both having the shape of the profile to be painted, but bigger than it by few millimetres.
Inside the container a depression is maintained, by means of a suction pump, in the range of 150–250 millibars, and paint is continuously introduced therein by means of a conduit. The term “vacuum” is here intended to mean a depression within that range of values or near such values. Normally the power absorbed by the suction pump is between 7 and 11 KW.
The air outside the container is sucked inside, at very high speed, through the slits existing between the profile which is passing through the container and the afore-mentioned inlet and outlet apertures. In this way, when the profile is inserted into the apertures of the container, the paint introduced therein does not exit from the apertures, because it is sucked in by the suction pump together with the air. The paint is then filtered in an autonomous filter assembly and introduced into the painting head again.
In this conventional painting head, the profiles are usually inserted one after the other, with the leading end of one in contact with the trailing end of the previous one, so as to prevent as much as possible the pressure inside the container from falling, due to a prolonged opening of one or both the inlet and outlet apertures.
This way of working also allows to prevent the leading end and the trailing end of each profile from being painted when this is not desired. In fact, if the following profile enters into the container containing the paint in contact with the trailing end of the profile which precedes it, neither the leading end of one nor the trailing end of the other come into contact with the paint. Obviously this only occurs when the leading end and the trailing end have mating and matching surfaces.
This conventional painting head has several disadvantages however.
First of all, it does not allow to insert one profile at a time into the container, because the afore-said inlet and outlet apertures, when they are not partly obstructed by the profile, cause a sudden increase in the pressure inside the container, up to values near those of atmospheric pressure, with a consequent loss of paint through the apertures, due to the decrease in the suction effect.
Moreover, when the profile has the leading end and the trailing end already worked and shaped, for example to make the frame of a piece of furniture, a painting, a window or door frame or suchlike, and the leading end and/or the trailing end do not have to be painted, the conventional painting head is not able to meet this requirement.
A further disadvantage of the conventional painting head is that, in the event of a sudden interruption in the energy feeding the suction pump, with a consequent increase in the pressure inside the container, the paint contained therein exits uncontrollably from the inlet and outlet apertures, even if there is a profile being worked inside. It is for this reason that below the conventional painting head it is necessary to put a tank or a collection container for the paint, which can contain it to prevent it from falling onto the floor or dirty other machinery nearby.
DE-A-19707157 discloses a device to dust moving flat products having a closed housing with a slotted outlet, a powder-dusting device located inside the housing and a suction device for surplus powder. The powder-dusting device is provided inside a suction chamber of the suction device. The suction chamber is surrounded by a blast chamber and the latter opens out through the slotted outlet of the housing. The powder-dusting device is provided with a nozzle extending especially into the region of the slotted outlet. This outlet and/or the outlet of the suction chamber can be sealed off by a slider, plate, grill or something similar. Also this known device has the above-mentioned disadvantages.
The present Applicant has devised and embodied this invention to overcome these and other shortcomings of conventional painting heads, and also to obtain further advantages, as will be explained hereafter.